The spine or vertebral column comprises a plurality of separate vertebrae. The vertebrae are movable relative to one another, and separated from one another by fibrocartilage called inter-vertebral discs.
In its entirety, the spinal column is highly complex in that it houses and protects critical elements of the nervous system which have innumerable peripheral nerves and arterial and venous bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve immobilization by implanting artificial assemblies in or on the spinal column.
Inter-Vertebral Disc Disease
For a patient suffering from acute inter-vertebral disc (IVD) disease, it is desirable to induce fusion and distraction of the two adjoining vertebrae, adjacent to the affected inter-vertebral disc. Inter-vertebral disc disease may particularly affect the cervical region of the spine. By distracting the two adjoining vertebrae, this relieves pressure on the spinal nerves and/or spinal cord.
Fusion cages are presently the preferred device for inducing fusion and distraction between adjoining vertebrae. Fusion devices provide a means of opening disc collapsed space between opposing vertebrae, relieving pressure on the nerves and/or spinal cord, by effectively placing a spacer in the disc space. Presently used spinal cages are generally hollow cuboid shaped devices.
Spinal Fixings
In order to treat certain injuries or conditions of the spinal column a metal rod that is bendable to match the natural curvature of the spine is mechanically attached at strategically selected vertebrae, allowing the rod to be rigidly fixed to the spine. This provides a rigid support to the spinal column. For this, screws located in the bone structure are typically fixed to a specially designed clamp to attach to a spinal rod. A problem with specially designed clamps is that bone structure cannot be determined until the patient's bone is exposed, therefore it is not known what angle the screw will need to be relative to the rod until the spinal bone structure has been examined. Certain parts of the vertabra cannot stably receive a bone screw, or can only receive a bone screw screwed in at a certain angle. Also, for spinal fixings for small animals, within the confined spaces allowed therein, conventional rod anchoring methods are not suitable since the placement of the pedicle screw and the direction of the rod cannot be matched adequately. A system that allows for easy adjustment of the alignment of a bone screw relative to a rod is needed. In particular, a system that can be used in small animals is needed, wherein confined spaces make conventional rod anchoring systems unsuitable.
Lumbo-Sacral Fusion
In vertebrates, the sacrum is a large, triangular bone at the base of the spine. The lumbo-sacral joint is a joint between the sacrum and the vertebra that is adjacent the sacrum. As between other vertebra, there is an intervertebral disc between the sacrum and the adjacent vertebra. For the purposes of this invention, the sacrum is considered to be part of the spine.
Compression of the nerves passing through the lumbo-sacral joint can occur, which is a symptom of lumbo-sacral disease.
A system for distracting the lumbo-sacral joint and stabilising the joint is needed. The bone structure of the sacrum may be soft. Perpendicular placement of a bone screw is often not possible. Therefore, there is a need for multi-axial positioning of a bone screw with respect to a stabilising rod. Many devices for providing multi-axial positioning of a bone screws with respect to a stabilising rod loosen over time, providing an unstable joint. Also, if the bone screw becomes loose, this presents forces on the fixing device, which may loosen due to the forces. There is therefore a need for a fixing device that overcomes one or more of these problems, and particularly for lumbo-sacral fusion.